1. Field of the Invention
This invention relates to a process for enhancing the conductivity of substituted and unsubstituted polyaniline. In particular, polyaniline coated textile fibers are heated for a brief period of time to achieve a significant increase in conductivity.
2. Prior Art
Aniline is known to polymerize into at least five different forms of polyaniline, e.g., leucoemeraldine, protoemeraldine, emeraldine, nigraniline and pernigraniline. Not all of the forms of polyaniline are conductive. However, after suitable doping with a counter ion, the emeraldine form has demonstrated relatively good conductivity and has been the focus of extensive research
The conductivity of the emeraldine base form of polyaniline and 50% protonated emeraldine hydrochloride was found to increase when exposed to about four torr of water vapor. Angelopaulos, et al. "Polyaniline: Processability from Aqueous Solutions and Effect of Water Vapor on Conductivity", Synthetic Metals, 21, (1987), pp. 21-30. Additional research on influence of moisture on conductivity was reported by Javadi, H., "Conduction Mechanism of Polyaniline: Effect of Moisture", Synthetic Metals 26 (1988), pp. 1-8.
The effect of temperature on the stability of polyaniline in terms of electrical conductivity has been the subject of prior investigation. In Wei, et al. "Thermal Analysis of Chemically Synthesized Polyaniline and Effects of Thermal Aging on Conductivity", Journal of Polymer Science, Vol. 27 (1989), pp. 4351-63, thermal aging was found to lead to deterioration in the conductivity of hydrochloric acid doped polyaniline. However, redoping the polyaniline with hydrochloric acid partially reversed the degradation. Various other investigators have found an accelerated loss of conductivity upon subjecting polyaniline to increased temperatures: Neoh, et al. "Stability Studies of Polyaniline", Polymer Degradation and Stability 27 (1990), pp. 107-117, hydrochloric acid doped polyaniline shows a decrease in conductivity after exposure to temperatures above 60.degree. C.; Hagiwara, T., et al. "Thermal Stability of Polyaniline", Synthetic Metals 25 (1988) pp. 243-252 the conductivity of hydrochloric acid doped polyaniline decreased while the sample was held at 150.degree. C. The crystallinity of doped and undoped polyaniline was found to increase at temperatures up to 150.degree. C. Fosong, W., et al. "Study on the Crystallinity of Polyaniline", Mol. Cryst. Liq. Cryst., Vol 160 (1988), pp. 175-184. Fosong, et al. also found that use of small molecular sized dopants, such as HCl, lead to higher crystallinity.
Similarly, prior art references addressing the thermal stability of polypyrole indicate a decrease in conductivity after heat treatment. Kuhn, U.S. Pat. No. 4,803,096 and Munstedt, "Ageing of Electrically Conducting Organic Materials" Polymer 29(2) (1988) pp. 296-302.
The desire to form compositions of polyaniline and thermoplastic polymers has led to the development of more thermally stable polyanilines. In particular, polyaniline doped with p-toluene sulfonic acid is disclosed as suitable for melt processing with thermoplastic polymers in International Patent Application Number PCT/US88/02319 by Jen et al., entitled "Thermally Stable Forms of Electrically Conductive Polyaniline". Examples of suitable polymers and their typical melt processing temperatures are: polyethylene, 400.degree.-450.degree. F.; polypropylene, 450.degree.-500.degree. F.; nylon 6, 450.degree.-500.degree. F.; nylon 6,6, 500.degree.-600.degree. F.; polyethylene terephthalate, 500.degree.-600.degree. F., and polyester, 500.degree.-600.degree. F. Other references showing a relationship between the dopant selected and the thermal stability of the doped polyaniline are Kulkarni et al , "Thermal Stability of Polyaniline", Synthetic Metals 30, pp. 321-325 (1989) and Elsenbaumer et al., "Stability of Doped Conducting Polymers", Allied-Signal, Inc., Corporate Research, Morristown, N.J.